Backplane power distribution system having impedance variations in the form of spaced voids

ABSTRACT

A powerplane for use in a backplane power distribution system. The backplane includes a conductive sheet for distributing power from a power source to a load. The powerplane further includes source locations and load locations for coupling the conductive sheet to a power source and a load. The conductive sheet is provided with impedance variations for balancing the resistance of the conductive sheet between the source locations and load locations. The impedance variations are spaced apart voids with the separation between the voids decreasing towards the source locations, thereby promoting even distribution of current to all the load locations.

FIELD OF THE INVENTION

The present invention relates generally to computer system backplanesand, more particularly, to powerplanes for distributing power inbackplanes.

BACKGROUND OF THE INVENTION

A computer system backplane typically is a multilayer substratecomprising a plurality of conductive layers interweaved with a pluralityof dielectric layers. The backplane carries a plurality of parallelmultiterminal sockets that receive in an edgewise manner circuit boardson which computer system components are constructed. Some of thebackplane conductive layers are used for signal propagation. Otherconductive layers are used to distribute the power necessary for systemoperation. These conductive layers are known in the art as powerplanesand are generally in the form of solid sheets of conductive materialsuch as copper.

Each multiterminal, socket typically includes a plurality of pins whichpass through small, plated vias bored through the layers of thebackplane. Each pin makes contact with a desired one of the backplaneconductive layers. Where no connection to a particular conductive layeris desired, a region surrounding the via through that conductive layeris insulated to prevent the pin from making contact. The plated vias aresized relative to the connector pins for a press fit. Power supplyconnections are made in a generally similar manner.

In each powerplane, some of the plated vias make contact with load pins,i.e., pins coupled to the circuit boards received by the sockets. Othervias are connected to source pins coupled to a power supply.

It will be appreciated that due to design constraints the source pinsare not always centered between the load pins, leading to unequaldistribution of current over the powerplanes and unequal current sharingamong the load pins. For example, load pins having a shorter lineardistance to the source pins will have a lower resistance with respect tothe source pins and thus will source more current than load pins furtherfrom the source pins. To avoid exceeding the current rating of the loadpins closest to the power source, smaller power levels are required.This results in inefficient use of the current sourcing capacity of thefor distant load pins.

An attempt to equally distribute current has been made using a steppedbackplane configuration. See U.S. Pat. No. 4,450,029 to Holbert et al.In a stepped backplane, the conductive and dielectric layers arelaminated while having the same transverse extent. An edge of thebackplane is then milled to expose the conductive layers in a steppedfashion. Rectangular bus bars are then mounted to the exposed conductivelayers to provide a parallel power distribution. The step backplanehowever fails to provide equal current over the length of thepowerplanes and thus fails to provide equal current to each load pin.Moreover, stepped backplanes are costly as they require post laminationmilling.

Consequently, there exists in the industry a need to provide a costeffective method for evenly distributing current to the load pins of apowerplane. The present invention addresses this need as well as otherneeds.

SUMMARY OF THE INVENTION

The present invention is a powerplane for use in a backplane powerdistribution system. The backplane includes a conductive sheet fordistributing power from a power source to a load. The powerplane furtherincludes source locations and load locations for coupling the conductivesheet to a power source and a load. The conductive sheet is providedwith impedance variations for balancing the resistance of the conductivesheet between the source locations and load locations, thereby promotingeven distribution of current to the load locations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectioned, perspective view of a conventional backplane witha thickness greatly exaggerated relative to the transverse dimensions;and

FIG. 2 is a plan view of an exemplary powerplane conductive layer of abackplane showing the structural features of an embodiment the inventionto vary the resistance along the dimensions of the backplane.

FIG. 3 is a perspective view of a circuit board coupled to thepowerplane.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, and more particularly to FIG. 1, there isshown a conventional backplane 10 for use in a system requiring powerdistribution, e.g., a computer system. Backplane 10 is typically alaminated structure comprising a plurality of alternating conductivelayers 12a-i and interweaved dielectric layers 14ab, 14bc, . . . and14hi. The conductive layers may be implemented with copper, gold,silver-palladium, alloy, tungsten, etc. The dielectric layers may befiberglass-epoxy composites. It is noted that the conductive anddielectric layers have been numbered such that each conductive layer hasa single letter associated with it and each dielectric layer hasassociated therewith the two letters that are associated with theimmediately neighboring conductive layers. The conductive layers may beless than 50 microns thick. Thus, it is important to note that thethickness of backplane 10 has been exaggerated in order to show theconductive and dielectric layers clearly.

Backplane 10 provides electrical communication between a power sourceand various functional units or loads. For example, in a computersystem, the functional units may be circuit boards carrying electricalcomponents. Some of the conductive layers 12a-i may be signal layers forsignal propagation while other layers may be powerplanes for providingparticular direct current (DC) voltage levels to the functional units.For example, in computer systems, the DC voltage levels are typically 5volts, 3.3 volts, or even lower. In the exemplary embodiment, conductivelayers 12a and 12i are signal layers and layers 12b-h are powerplanes.

Backplane 10 is provided with an array of locations for coupling thebackplane 10 to functional units. For example, the locations may beconnector straps or pads, wiring networks, etc. In the exemplaryembodiment, the locations are holes or vias 15 which receive load pinsand source pins. The formation of vias 15 is described more fullyhereinbelow. The number of load near load locations 20 and distant loadlocations 22 and the number of source locations 30 vary according to theenvironment in which backplane 10 is used. In computer systems, thenumber of load locations 20 and 22 depends on the number of circuitboards to be plugged in backplane 10 as well as the power requirementsof the circuit boards and the current carrying capability of the pins.The number of source locations 30 depends on similar variables. Loadlocations 20 and 22 and source locations 30 extend through the layers ofbackplane 10 and into sockets mounted on backplane 10. The sockets mayinclude resilient contacts for coupling the contacts on a functionalunit, e.g., a circuit board or a power supply, load locations 20 and 22or source locations 30. FIG. 3 illustrates a circuit board 52 coupled toa socket module 50 which is connected at load locations 20 to thepowerplane 40.

In the exemplary embodiment, load locations 20 and 22 and sourcelocations 30 are press fit into backplane 10. In alternativeembodiments, the load locations 20 and 22 and the source locations 30may be soldered or bolted down to the backplane 10, or the functionalunits may contain the pins for insertion into the array of vias 15provided by backplane 10. Vias 15 may be plated so that a correspondingpin makes contact with a desired one of the backplane conductive layers,whether a signal layer or a powerplane, as shown as 18b and 18h on FIG.1 where plated vias contact conductive layer 12b and 12h, respectively.Where connection to a given layer is not desired, a region may beprovided surrounding the hole through that particular conductive layerto insulate the conductive layer from the pin. Thus, backplane 10 has anarray of vias or locations passing through the powerplanes but makingelectrical contact with only certain conductive layers.

FIG. 2 illustrates an exemplary powerplane 40 having load locations 20,22 and source locations 30 for receiving load pins and source pins fordistributing power from a power source to functional units. Powerplane40 is provided with the array of vias 15 as mentioned a plurality ofresistances or impedance variations 42. In powerplanes there is verylittle inductance, so the terms resistance and impedance are usedinterchangeably. Balancing the resistance is the main objective becauseby doing so, this will balance the DC current. It is also true, however,to say that the impedance is balanced because both the inductive partand the resistive part of the impedance will be balanced. Impedancevariations 42 balance the resistance of powerplane 40 between sourcelocations 30 and load locations 20, 22 as described hereinbelow. Asshown in the exemplary embodiment of FIG. 2, impedance variations 42 maybe rectangular voids in the conductive sheet comprising powerplane 40and may be provided in multiple rows, extending parallel to loadlocations 20 and 22 and disposed between load locations 20, 22 andsource locations 30. Furthermore, in the exemplary embodiment, impedancevariations 42 are spaced more closely near source locations 30 with thespacing gradually increasing as impedance variations 42 move furtherfrom source locations 30, i.e., d₁ is less than d_(x+1) is less thand_(n-1) is less than d_(n), as illustrated in FIG. 2.

It should be appreciated that the resistance between a load location 20,22 and a source location 30 is a function of the resistivity anddimensions of the conductive material between the pins. In conventionalpowerplanes, i.e., those without impedance variations 42, load locations22 further from source locations have a higher resistance than loadlocations 22 closer to source locations. This results from the increasedlength of the conductive material. By providing impedance variations 42having an increasing spacing as described above, the resistance betweensource locations 30 and near load locations 20 increases relative to theresistance between source locations 30 and distant load locations 22.This provides a more even load location-to-source location resistancefor all load locations without changing the design of the locationlayout. Accordingly, current is shared more evenly between loadlocations 20, 22; and the voltage difference between distant loadlocations 22 and near load locations 20 is reduced to near zero. As aresult, the functional unit receives a stable voltage level for morereliable operation.

It is noted that the characteristics and/or location of impedancevariations 42 may vary, provided the resistance between load locations20, 22 and source locations 30 is substantially the same for all loadlocations 20, 22. For example, impedance variations 42 may be circularor noncircular, and the location of impedance variations 42 need not runsubstantially parallel to the load pins. In addition, impedancevariations 42 may be provided singly or in multiple rows. Moreover,rather than being voids in powerplane 40, impedance variations 42 may becomprised of nonconductive material, or even conductive material havinga resistivity greater than the resistance of the surrounding sheet,provided that in the later instance, the spacing between impedancevariations 42 decreases as impedance variations 42 move away from sourcelocations 30. In other embodiments, impedance variations 42 may comprisevariations in the thickness of powerplane 40. For example, powerplane 40may be thinner near source locations 30 and gradually thicken as itmoves further from source locations 30.

Some of the above described structural features may be better understoodwith reference to an exemplary fabrication sequence. The first stepinvolves providing the conductive and dielectric layers for lamination.These layers may be provided as separate conductive and dielectriclayers, or as a number of composite or dielectric only layers. Compositelayers may include a copper-dielectric composite or acopper-dielectric-copper composite. In the latter composite, thedielectric layer is prepreg material having outer gel-cured layers andan inner completely cured layer.

Prior to lamination of all the layers to form the backplane, eachconductive layer 12a-i is etched with an appropriate pattern. As notedabove, load locations 20, 22 and source locations 30 typically passthrough all the conductive layers 12a-i. Where a connection of alocation to a given layer is not desired, the etching step may removeappropriate material in the vicinity of a via 15 for the particularlocation. During the etching step, impedance variations 42 may also beetched. Advantageously, no additional expense would be incurred byproviding impedance variations 42 during etching. It is noted thatimpedance variations 42 may alternatively be provided, for example, byusing a release agent such as silicone-impregnated tape or by providinga conductive material having a different resistivity than that ofsurrounding powerplane 40.

After etching, the various layers are laminated under heat and pressureto form a single rigid assembly. Vias 15 for load locations 20, 22 andsource locations 30 may then be drilled through the rigid laminatedstructure and plated to facilitate contact to a desired conductivelayer.

It will, of course, be understood that various modifications andadditions can be made to the embodiments discussed herein above withoutparting from the scope or spirit of the present invention. For example,the powerplane may be employed in other systems requiring powerdistribution, such as telecommunications systems, local area networks,programmable logic controllers, etc. Accordingly, the scope of thepresent invention should not be limited to the particular embodimentsdiscussed above, but should be defined only by full and fair scope ofthe claims set forth below.

What is claimed is:
 1. A powerplane for use in a backplane powerdistribution system, comprising:a conductive sheet coupled to at leastone source location and a plurality of load locations, said conductivesheet having an arrangement of a plurality of resistances to distributesubstantially the same amount of current from said at least one sourcelocation to each of said plurality of load locations wherein theseparation between said plurality of resistances on said conductivesheet decreases as said resistances near said source locations.
 2. Apowerplane according to claim 1, wherein said plurality of resistancescomprise a plurality of voids arranged so that the spacing between thevoids decreases as said voids near said source locations.
 3. Apowerplane according to claim 2, wherein said voids are arrangedsubstantially parallel to said load locations.
 4. A powerplane accordingto claim 2, wherein said voids are arranged in multiple rows.
 5. Apowerplane according to claim 2, wherein said voids are formed by aprocess including etching.
 6. A powerplane according to claim 2, whereinsaid voids are rectangular.
 7. A powerplane according to claim 3,wherein said conductive sheet includes a first edge, a second edge, anupper edge, and a lower edge, said source locations being disposedproximate said lower edge and said first edge, said load locations beingdisposed proximate said second edge, said voids being disposed betweensaid source locations and said load locations, said voids extending froman area proximate said lower edge to an area proximate said upper edge.8. A backplane power distribution system for distributing power from apower source, comprising:a laminate havinga plurality of interleaveddielectric layers and conductive layers wherein at least one of saidconductive layers is used as a powerplane for distributing said power;and a plurality of source locations and load locations, said sourcelocations being provided to couple said powerplane to said power sourceand said load locations being provided to couple said powerplane to atleast one load said powerplane having a plurality of resistances,whereinsaid resistances comprise voids spaced apart with a spacing whichdecreases as said voids near said source locations.
 9. A backplaneaccording to claim 8, wherein said voids are disposed in a row extendingin a direction substantially parallel to a second row having said loadlocations.
 10. A powerplane for use in a backplane power distributionsystem comprising:a conductive sheet coupled to at least one sourcelocation and a plurality of load locations, said conductive sheet havingan arrangement of a plurality of voids arranged so that the separationbetween said plurality of voids on said conductive sheet decreases assaid voids near said source locations.
 11. A powerplane according toclaim 10, wherein said voids are arranged substantially parallel to saidload locations.
 12. A powerplane according to claim 10, wherein saidvoids are arranged in multiple rows.
 13. A powerplane according to claim10, wherein said conductive sheet includes a first edge, a second edge,an upper edge, and a lower edge, said source locations being disposedproximate to said lower edge and said first edge, said load locationsbeing disposed proximate to said second edge, said voids being disposedbetween said source locations and said load locations, said voidsextending from an area proximate to said lower edge to an area proximateto said upper edge.
 14. A backplane power distribution system fordistributing power from a power source, comprising:a laminate havingaplurality of interleaved dielectric layers and conductive layers whereinat least one of said conductive layers is used as a powerplane fordistributing said power, and a plurality of source locations and loadlocations, said source locations being provided to couple saidpowerplane to said power source and said load locations being providedto couple said powerplane to at least one load, said powerplane having aplurality of voids spaced apart with a spacing which decreases as saidvoids near said source locations to distribute current on saidpowerplane so the voltage difference between said load locations isreduced to near zero.
 15. A backplane according to claim 14, whereinsaid voids are disposed in a row extending in a direction substantiallyparallel to a second row having said load locations.